Modular power converter

ABSTRACT

A power converter includes at least one capacitor and at least two semiconductor power switches. Each capacitor includes a pair of connectors for connecting the capacitor to the at least two semiconductor power switches. Moreover, each capacitor includes at least one other pair of connectors in order to connect the capacitor to at least two semiconductor power switches or a direct current network.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP01/06326 which has an Internationalfiling date of Jun. 6, 2001, which designated the United States ofAmerica and which claims priority on German Patent Application number100 37 379.8-32 filed Aug. 1, 2000, the entire contents of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to a power converter,constructed using a multilevel circuit, for converting direct current toalternating or vice versa. The converter may include at least onecapacitor and at least two semiconductor power switches. The or eachcapacitor may have a connection pair for connection of the capacitor toat least two semiconductor power switches.

BACKGROUND OF THE INVENTION

Power converters using a multilevel circuit are known, for example, fromU.S. Pat. No. 5,737,201. This document describes in particular thetheoretical principles of a multilevel circuit. A multilevel circuitallows a power converter to be designed in a modular manner. Each of themodules includes at least two semiconductor power switches and at leastone capacitor, which is arranged between the power switches. The specialfeature of the multilevel circuit is that the capacitors are not all ofthe same potential, but may be related to different potentials(so-called floating capacitors). An intermediate circuit voltage ispassed to a number of floating capacitors in such a way that the voltageload on one semiconductor power switch is the result of the differentbetween the voltage on two capacitors.

According to the prior art, the capacitors which are used in powerconverters have only one connection pair. The connection pair isconnected to the semiconductor power switches in a first module. Thesemiconductor power switches in a further module are connected to theconnection pair of the capacitor via an additional electricalconnection. This additional electrical connection should be designed tohave as low an inductance as possible, in order to reduce the load onthe semiconductor power switches. U.S. Pat. No. 5,737,201 does notdescribe in any more detail the problem associated with the electricalconnection between the capacitors and the semiconductor power switcheshaving as low an inductance as possible.

An electrical connection with as low an inductance as possible can beachieved, in entirely general form, by special design measures. Forexample, a reduction in the size of an area through which a verticalcomponent of a commutation circuit of a module flows leads to theelectrical connection having a lower inductance. The inductance can bereduced considerably by using conductors that are as wide as possiblebetween the capacitors and the semiconductor power switches, and by thedistance between the forward conductor and the return conductor being asshort as possible. A low-inductance electrical connection between thecapacitors and the semiconductor power switches is subject to certainlimits, relating to the design, accuracy and life of the powerconverter.

In order to allow a low-inductance electrical connection which can bephysically implemented relatively easily between the capacitors and thepower switches, it is known from EP 0 944 163 A1 for the capacitors in apower converter to be subdivided into two, and for one half of onecapacitor to be associated with the semiconductor power switches of afirst module of the power converter, and for the other half of thecapacitor to be associated with the power switches of another module.Specific low-inductance electrical conductors, which are in the form ofrail packs (so-called busbars) are used to connect the capacitor halvesto the power switches of the associated modules. The rail packs have,for example, a laminated structure composed of two copper plates, whichare used as conductors, with a plate or sheet composed of a solidinsulator arranged between them. Owing to so-called partial dischargesbetween the copper plates, the solid insulator is subject to aging,which restricts the life of the rail pack. Furthermore, low-inductanceconductors in the form of rail packs cause problems in handling(mechanical loads on the rail packs can adversely affect their lowinductance), and they are very expensive.

A further disadvantage of the power converter which is known from EP 0944 163 A1 occurs with relatively low rating power converters, in whicha single module with one capacitor and two power switches would besufficient. Even in power converters such as these with only onecapacitor, the capacitor is subdivided into two capacitor halves, whichforces up the production costs for these known power converters.

SUMMARY OF THE INVENTION

One object of an embodiment of the present invention is to define and todevelop a power converter of the type mentioned initially such that itis possible to produce an electrical connection between the capacitorsand the power switches whose inductance is as low as possible, whoselife is as long as possible, and which is as cost-effective as possible.

In order to achieve this object against the background of the powerconverter of the type mentioned initially, the an embodiment of thepresent invention proposes that the or each capacitor has at least onefurther connection pair for connection of the capacitor to at least twosemiconductor power switches or to a DC network.

Thus, according to an embodiment of the present invention, at least onefurther connection pair is passed to the exterior on the or eachcapacitor. The capacitor can be connected via the one connection pair toat least two semiconductor power switches in a first module, and viaeach further connection pair to at least two further semiconductor powerswitches in further modules, or to a DC network. The connection pairsmay be passed out of the capacitor at any desired point. However, it isrecommended that the connection pairs be passed out where the designresults in further modules of the power converter being arranged.

The capacitors which are used in power converters normally have twocontact tracks, which run parallel to one another, in their interior,which contact tracks extend virtually over the entire capacitor lengthand between which a number of parallel-connected capacitor elements arearranged. Owing to the requirements for low inductance, these contacttracks may be in the form of low-inductance electrical conductors.Connection pairs can be passed out of the capacitor at virtually anydesired points, originating from the contact tracks. Low-inductanceelectrical connections, which are generally provided in any case incapacitors for power converters, are thus used as low-inductanceconductors for connection of the capacitors to the semiconductor powerswitches in the individual modules. Since there is no need for therelatively expensive rail packs (busbars) in the power converteraccording to the invention, the production and assembly costs of thepower converter can be reduced to a major extent.

One advantageous development of an embodiment of the present inventionproposes that the connection pairs of one capacitor be passed out ofthis capacitor on different sides of the capacitor. The capacitor may bephysically arranged between the modules to whose semiconductor powerswitches it is intended to be connected. This allows the power converterto have a particularly compact construction.

One preferred embodiment of the present invention proposes that the oreach capacitor have two connection pairs which are passed out of thiscapacitor on opposite sides of the capacitor. This allows the powerconverter to be formed from a number of modules arranged one above theother or alongside one another. This elongated structure of the powerconverter has the advantage that it is easy to see the individualcomponents, so that assembly and maintenance are simplified. Theelongated structure results in particular advantages when air is used tocool the semiconductor power switches, since the cooling air—in contrastto the situation in power converters whose construction is complex andangled—can be passed without any problems through the elongated powerconverter. Apart from being cooled by air, the power converter accordingto the invention may, of course, also be cooled in any other desiredmanner.

Each connection pair advantageously has two connecting contacts, eachhaving at least one connecting element, with the connecting elements ofeach connecting contact being connected via a low-inductance flat railconductor to a connection of a semiconductor power switch. In order toreduce the inductance of the connection between the capacitor and thepower switches, one connecting contact of one connection pair may have anumber of connecting elements running parallel to one another. The railconductors are composed, for example, of copper and are insulated byair. An isolator panel may be arranged between the forward and returnrail conductors, in order to avoid short circuits caused by foreignbodies and/or when air cooling is used, to carry the cooling air throughthe power converter, through its components. The isolator panel iscomposed, for example, of plastic.

In order to allow the cooling air to he routed in a particularly simplemanner along the capacitors, when air cooling is used, another preferredembodiment of the invention proposes that at least one cold plate bearranged at a distance from at least one outer face of the or eachcapacitor. The distance between the outer faces of the capacitor and thecold plate may be used as a cooling channel, through which the coolingair can be passed laterally along the capacitor.

The upper face and lower face of the or each capacitor advantageouslyhave areas which overhang the at least one outer face of the or eachcapacitor, and on which the at least one cold plate is mounted. This isof particularly simple structure for the cooling channels for thecooling air, but allows particularly effective air guidance.

The power converter is preferably in the form of an inverter, whichconverts a DC voltage to an AC voltage. Alternatively, the powerconverter according to the invention may also be in the form of a DCcontroller. The power converter according to the invention isparticularly suitable for use in the medium-voltage and high-voltageranges.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, application options and advantages of the presentinvention can be found in the following description of exemplaryembodiments of the invention, which are illustrated in the figures ofthe drawing. In this case, all the described or illustrated featuresform the subject matter of the invention in their own right or in anydesired combination, irrespective of their combination in the patentclaims or their referral back, and independently of their formulationand representation in the description and in the drawing. In thefigures:

FIG. 1 shows a simplified circuit diagram of a three-phase inverter;

FIG. 2 shows a simplified circuit diagram for one phase of a converterin the form of a multilevel circuit;

FIG. 3 shows the capacitor for one preferred embodiment of a powerconverter according to the invention, in a section view from the front;

FIG. 4 shows a plan view of the capacitor shown in FIG. 3; and

FIG. 5 shows one preferred embodiment of a power converter according tothe invention, constructed using a multilevel circuit, in a view fromthe front.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a circuit diagram of a inverter which is known from theprior art. A DC voltage U_d is applied to the input side of theinverter, and is converted by the inverter to an AC voltage, for exampleto a 3-phase voltage. The DC voltage U_d is applied to a capacitor 2 inthe inverter. The inverter has a series circuit comprising two powerswitches 3 for each phase of the AC voltage. Transistors, thyristors,GTOs (Gate Turn-Off Thyristors) or preferably IGBTs (Insulated GateBipolar Transistors) may be used, for example, as the power switches 3.The series circuits formed by the power switches 3 for the individualphases are arranged in parallel with one another and in parallel withthe capacitor 2, so that the DC voltage U_d is also applied to theseries circuits formed by the power switches 3. The DC voltage U_d isconverted to the 3-phase voltage by cyclic opening and closing of thepower switches 3. The frequency and the amplitude of the fundamental ofthe 3-phase voltage can be varied by driving the power switches 3 in aspecific manner. The 3-phase voltage may also be supplied to a load, forexample to a three-phase motor 4.

FIG. 2 shows a circuit diagram for one phase of a power converter 1 inthe form of an inverter, according to an exemplary embodiment of thepresent invention. The inverter is designed in the form of a so-calledmultilevel circuit. Power converters 1 using a multilevel circuit areknown, for example, from U.S. Pat. No. 5,737,201 where, in particular,the theoretical principles of multilevel circuits and embodiments ofpower converters using multilevel circuits, but which are not used inpractice, are described. The production of a power converter 1 using amultilevel circuit may result in design problems, since the electricalconnections between the capacitors 2 and the power switches 3 should bedesigned to have as low an inductance as possible. One possible way toproduce a power converter 1 using a multilevel circuit is known from EP0 944 163 A1. Reference is expressly made to these two documents.

A multilevel circuit allows the power converter 1 to be designed in amodular manner. In the present exemplary embodiment, the power converter1 is in the form of a 4-level circuit, that is to say it has threemodules for each phase. Each of the modules has two semiconductor powerswitches 3 and one capacitor 2, arranged between the power switches 3.In the exemplary embodiment shown in FIG. 2, the entire DC voltage U_dis applied to the capacitor C1, ⅔ of the DC voltage U_d is applied tothe capacitor C2, and ⅓ of the DC voltage U_d is applied to thecapacitor C3.

The special feature of the multilevel circuit is that the capacitors 2are not all of the same potential, but can be referred to differentpotentials (so-called floating capacitors). A module intermediatecircuit voltage is passed to a number of floating capacitors in such away that the voltage load on a semiconductor power switch 3 is thedifference between the voltage on two capacitors 2.

FIG. 3 shows a capacitor 2 as is used in the power converter 1 accordingto the invention, in a section view from the front. The capacitor 2 hasa housing comprising an upper face 5, a lower face 6 and outer faces 7.Two contact tracks 8 run parallel to one another along the outer faces 7in the interior of the housing 5, 6, 7. The contact tracks 8 extendvirtually over the entire length 1 of the capacitor 2. A number ofcapacitor elements 9 are arranged between the contact tracks 8 and,together, result in the capacitor 2 having the desired capacitance.

Owing to the requirements for low inductance within the capacitor 2, thecontact tracks 8 are in the form of low-inductance electricalconductors. Originating from the contact tracks 8, connection pairs canbe passed out of the housing 5, 6, 7 of the capacitor 2 at virtually anydesired points in the capacitor 2. However, it is recommended that theconnection pairs be passed out of the housing 5, 6, 7 where, by virtueof the design, further modules of the power converter 1 are arranged.The connection pairs each have a number of connecting contacts 10, 11.

In the present exemplary embodiment, two connection pairs 10, 11 arepassed out of the housing 5, 6, 7 of the capacitor 2 on opposite sides,to be more precise on the upper face 5 and on the lower face 6. Eachconnection pair has two connecting contacts 10, 11. In order to reducethe inductance of the electrical connection between the capacitor 2 andthe power switches 3, each connecting contact 10, 11 has a number ofconnecting elements 12, in the present case five (see FIG. 4).

The upper face 5 and the lower face 6 of the capacitor 2 project beyondthe outer faces 7. A cold plate 13 is mounted externally on theprojecting areas, so that cooling channels 14 are formed between theouter faces 7 and the cold plates 13, through which cooling air can bepassed along the sides of the capacitor 2.

FIG. 5 shows a power converter 1 according to the invention. Theillustrated components of the power converter 1 are the components, asillustrated in FIG. 2, of the circuit for one phase of the powerconverter 1. The power converter 1 has a housing 15. A capacitor 2 andtwo semiconductor power switches, which are combined to form a powerswitch unit 16, are arranged alternately one above the other in thehousing 15. A heat sink 17, which is provided with cooling ribs, ismounted on each side of the power switch unit 16. The cooling ribs onthe heat sink 17 extend in the same direction as the cooling channels 14of the capacitors 2. The dimensions of the capacitors 2 are governedessentially by the capacitances and by the voltage of the capacitors 2,and hence also by the number of capacitor elements 9. For example, thecapacitor C1 is physically larger than the capacitor C2, and thecapacitor C2 is in turn physically larger than the capacitor C3.

The modular design of the power converter 1 can be seen particularlywell in FIG. 5. The compact modular design of the power converter 1according to the invention is only feasible at all by virtue of thedesign, as described above, of the capacitors 2 with a connection pair10, 11 on the upper face 5 and a further connection pair 10, 11 on thelower face 6 of the capacitor 2. The electrical connection between theconnection pairs 10, 11 of the capacitors 2 and connections 20 of thepower switch units 16 or a DC network is produced by way of flat,low-inductance rail conductors 18. The rail conductors 18 are composedof an electrically conductive material, for example of copper, and inthe present exemplary embodiment are insulated by air. There is no needto use expensive rail packs (so-called busbars)—as is normal in theprior art—for the power converter 1 according to the invention. Apartfrom being insulated by air, the rail conductors 18 may, of course, alsobe insulated by any other materials.

In order to avoid a short circuit between two adjacent rail conductors18 in an electrical connection resulting from foreign bodies enteringthe housing 15 (for example a forgotten tool, insects, animals) or thelike, an isolator panel 19 can be inserted between two adjacent railconductors 18. The isolator panels 19 are represented by dashed lines inFIG. 5 and consist of a solid insulator, for example composed ofplastic.

The power converter 1 according to the invention requires only a minimumnumber of additional low-inductance flat rail conductors 18 forconnection of the capacitors 2 to the semiconductor power switches 3,since the majority of the electrical connections between the capacitors2 and the power switches 2 are provided by the low-inductance contacttracks 8 within the capacitors 2.

The modular design of the power converter 1 according to the inventionallows the components (in particular the capacitors 2 and the powerswitch elements 16) of the power converter 1 to be assembled andmaintained particularly cost-effectively. The air-insulated,low-inductance, flat rail conductors 18 have a particularly long life,and can also transport high voltages without any problems. By virtue ofits design, the power converter 1 according to the invention is suitablefor use in the medium-voltage and high-voltage ranges. Furthermore, thecomponents of the power converter 1 according to the invention can becooled in a simple manner and particularly effectively by way of coolingair, by virtue of the modular design.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A power converter, constructed using a multilevelcircuit, for converting direct current to alternating current or viceversa, comprising: at least two semiconductor power switches; and atleast one capacitor having a connection pair connecting the at least onecapacitor to the at least two semiconductor power switches, and at leastone further connection pair for connection of the at least one capacitorto one of another at least two semiconductor power switches and a DCnetwork.
 2. The power converter as claimed in claim 1, wherein theconnection pairs of the at least one capacitor are passed out of the atleast one capacitor on different sides of thereof.
 3. The powerconverter as claimed in claim 1, wherein the at least one capacitor is aplurality of capacitors, and each capacitor of the plurality ofcapacitors has two connection pairs which are passed out of thecapacitor on opposite sides thereof.
 4. The power converter as claimedin claim 1, wherein each connection pair has two connecting contacts,the connecting contacts each having at least one connecting element,with the at least one connecting element of each connecting contactbeing connected via a low-inductance flat rail conductor to a connectionof a semiconductor power switch.
 5. The power converter as claimed inclaim 1, wherein at least one cold plate is arranged at a distance fromat least one outer face of the at least one capacitor.
 6. The powerconverter as claimed in claim 5, wherein an upper face and a lower faceof the at least one capacitor have areas which overhang the at least oneouter face of the at least one capacitor and on which the at least onecold plate is mounted.
 7. The power converter as claimed in claim 1,wherein the power converter is in the form of one of an inverter and aDC controller.
 8. The power converter as claimed in claim 2, whereineach connection pair has two connecting contacts, the connectingcontacts each having at least one connecting element, with the at leastone connecting element of each connecting contact being connected via alow-inductance flat rail conductor to a connection of a semiconductorpower switch.
 9. The power converter as claimed in claim 3, wherein eachconnection pair has two connecting contacts, the connecting contactseach having at least one connecting element, with the at least oneconnecting element of each connecting contact being connected via alow-inductance flat rail conductor to a connection of a semiconductorpower switch.
 10. The power converter as claimed in claim 2, wherein atleast one cold plate is arranged at a distance from at least one outerface of the at least one capacitor.
 11. The power converter as claimedin claim 3, wherein at least one cold plate is arranged at a distancefrom at least one outer face of each capacitor of the plurality ofcapacitors.
 12. The power converter as claimed in claim 4, wherein atleast one cold plate is arranged at a distance from at least one outerface of the at least one capacitor.
 13. The power converter as claimedin claim 11, wherein an upper face and a lower face of each of theplurality of capacitors have areas which overhang the at least one outerface of each of the plurality of capacitors and on which the at leastone cold plate is mounted.
 14. The power converter as claimed in claim2, wherein the power converter is in the form of one of an inverter anda DC controller.
 15. The power converter as claimed in claim 3, whereinthe power converter is in the form of one of an inverter and a DCcontroller.
 16. The power converter as claimed in claim 4, wherein thepower converter is in the form of one of an inverter and a DCcontroller.
 17. The power converter as claimed in claim 5, wherein thepower converter is in the form of one of an inverter and a DCcontroller.
 18. The power converter as claimed in claim 6, wherein thepower converter is in the form of one of an inverter and a DCcontroller.
 19. A power converter, comprising: at least one capacitorhaving a plurality of connection elements in contact with a plurality ofpower switch units, a first pair of the plurality of connection elementsbeing connected to a first power switch unit of the plurality of powerswitch units, and a second pair of the plurality of connection elementsbeing connected to a second power switch unit of the plurality of powerswitch units, wherein each power switch unit includes at least twoindividual power switches.